Linear regulation is a well known technique for producing a stable DC voltage from a raw, or unregulated DC supply. The unregulated supply could be, for example, a rectified and filtered AC source or an electrochemical battery. The unregulated supply must have a voltage higher than the intended output of the regulator since only a lower voltage can be produced using linear techniques. Additionally, the current output of the regulator may be limited as a safety measure, or to avoid damaging a sensitive electronic device connected to the regulator.
Linear regulators have two main components; a pass device and a feed back control circuit. The pass device is typically a transistor which is electrically located in series between the unregulated supply and the load. Current delivered to the load passes through the transistor, and is regulated by the control circuit. In this sense, the transistor acts as a variable resistor, and the effective resistance is controlled by the control circuit such that output voltage is maintained at a predetermined level. If the load is a time varying load, then the control circuit causes the pass transistor to vary linearly with the load.
One of the chief obstacles to overcome when using linear regulation techniques is power dissipation. The power dissipated by the pass transistor is the product of the difference between the unregulated voltage input and the output voltage, and the current through the device; that is, (V.sub.in --V.sub.out).multidot.I.sub.load. With regulators that are not current limited, the pass transistor may burn out if load demand is excessive. Providing a current limit action with the regulator can eliminate this threat in some systems, but in smaller systems, such as in battery powered portable electronics, the limited space available restricts the use of for example, a heat sink.
Pass transistor power dissipation is especially pertinent in portable electronic devices operated in volatile atmospheric conditions. These devices must be constructed so that even under extreme failure conditions the device will not ignite the surrounding atmosphere. Examples of applications where such safety measures are required include mining, grain processing, and chemical manufacturing facilities. Electronics operated in these areas must neither allow a sufficiently energetic spark to occur, nor may any exposed surface be heated to an unsafe temperature. In portable systems using rechargeable battery packs, the battery is a significant potential ignition source. The output of the battery is often regulated so as to limit the voltage, current, and total power available to the device. Since linear regulators are fairly simple, they lend themselves to the design of safe devices, provided that the heat issue is addressed.
Accordingly, there is a need to keep the pass transistor from over heating, and creating a potentially unsafe condition. However, since the space available in such systems is very limited, a means other than a heat sink is desirable to limit the power dissipation of the pass transistor. One such method is to provide a regulator. FIG. 1 illustrates a circuit diagram of a typical regulator according to the prior art. Unregulated DC source 10 provides a raw voltage and current Io to be delivered to load 12. The current passes through sense resistor 24 and pass transistor 18, which may be either a MOSFET or a bipolar transistor, as illustrated. Control circuit 22 is provided to control pass transistor 18 such that the output voltage V1 is constant and output current level Io is limited to a predetermined limit. As a result, there is a voltage drop V2 across pass transistor 18. Additionally, sense resistor 24 produces voltage drop V3 proportional to current through it.
Voltage drop V3 is fed to control circuit 22 via line 26 as a feed back signal. Control circuit 22 is resistively coupled to pass transistor 18 by resistor 28 and controls the effective resistance of pass transistor 18 to achieve the desired output voltage level so long as current level Io is below the predetermined limit. Should the current level reach the limit, control circuit 22 will cause V1 to drop until the demand of load 12 diminishes to agree with the limited output current level. Should the output be shorted by a conductor, the output voltage drops to zero, causing all of the source voltage to be evident across pass transistor 18, and the output current rises the limit level.
Once the current limit is reached and V1 begins to drop, voltage across pass transistor 18 increases. Since current Io is limited to a preset limit, when output voltage V1 drops, the power dissipation of pass transistor 18 rises proportionally as V2 increases. To keep pass transistor 18 from burning up, a shutoff circuit has been included. The shutoff circuit is comprised of transistor pair 39, arranged in a darlington configuration, capacitor 32, and resistor 34. As V2 increases, capacitor 32 begins charging at a rate controlled by resistor 84, which is quite large in value to achieve a delay effect. If the magnitude of voltage drop V2 is sufficient and lasting, the voltage across capacitor 32 will be enough to trigger transistor pair 30 to switch current into the bias leg of pass transistor 18. This current overwhelms the effect of control circuit 22 and pass transistor 18 turns off. A darlington pair configuration is used to overcome the effect that resistor 34 would have on a single switch transistor which has a much lower gain than the darlington pair 30.
The problem with the circuit illustrated in FIG. 1 is that the maximum sustained V2 cannot exceed the trigger voltage of transistor pair 80. The equivalent power dissipation of pass transistor 18 when this trigger voltage is evident across pass transistor 18 is typically well below its power rating, even at maximum current output. Also, should DC source 10 increase to a voltage higher than the pre-selected output voltage of the regulator by more than the trigger voltage of darlington pair 30, the regulator will be shut off. This is easily possible in a battery powered system where the battery is being recharged and the battery voltage rises to a level beyond its maximum non-charging voltage.
Therefore, there exists a need for a circuit in which a regulator can allow a sustained voltage drop across it's pass transistor so long as the resulting voltage drop does not cause excessive power dissipation. The circuit should also shut off the pass transistor should the voltage across it cause excessive power dissipation.